The present invention relates to an improved process for bleaching mechanical pulps, in which the unbleached pulp is subjected to flotation before bleaching.
The mechanical defiberization of wood produces high yields of pulps for paper production. The classical process is based upon an invention of the German scientist Keller in the first half of the 19th century. Debarked logs are pressed parallel to the fibre orientation against a rotating rough grindstone. This mechanically separates the fibers. The resulting "mechanical pulp" has approximately the brightness of the wood used initially and can be used for various kinds of paper.
The common term used in the industry is "groundwood" with the abbreviation "GW". Normally the timber is coniferous wood because of its longer and stronger fibers. Small-sized spruce timber from thinning work, with a trunk diameter of up to 15 cm, is the type most commonly used in Central Europe. Poplar is also occasionally used to produce mechanical pulps.
The classical groundwood pulp has been supplemented by a number of similar but differently produced wood pulps. The defiberization conditions are improved if the process is conducted at a higher pressure and a higher temperature level. At higher temperatures the lignin of the wood becomes softer and longer fibers with better strength properties are the result. The abbreviation used for this is "PGW" (pressurized groundwood).
Both processes produce coarse rejects as by-products (fiber bundles, slivers, shives) which have to be reground. Normally disk-refiners are used for the final defiberization process.
If logs are not available, saw mill waste can be chipped and the chips defiberized in refiners. This mechanical pulp is called "RMP" (refiner mechanical pulp).
Defiberization in refiners at a temperature of 120.degree. C.-140.degree. C. yields very good strength properties. The temperature treatment softens the lignin. The mechanical process at the edges of the refiner disks results in a very high amount of long fibers and a relatively low short fiber fraction. These wood pulps are called "TMP" (thermo mechanical pulp). The strength properties of a TMP are significantly superior to those of a standard groundwood. The higher temperatures of the defiberization conditions cause a darkening of the resulting pulp.
Prior to thermo mechanical defiberization, a chemical pretreatment of the wood chips is possible. A wide variety of different mechanical pulps are the result. The addition of sodium sulfite and caustic soda chemically modifies the wood and facilitates the defiberization process. Depending on the amount of chemicals, the treatment temperature and the intensity of the treatment with chemicals, wood pulps with very different properties and yields are obtained. These pulps are labeled with abbreviations such as "CTMP" (chemo thermo mechanical pulp) and "CMP" (chemo mechanical pulp).
Very high temperatures are commonly used now in producing wood pulp. Disintegration of wood to produce groundwood (GW) is done at temperature above 90.degree. C. Pressurized groundwood (PGW) is produced at circulation temperatures near the boiling point. The temperature level is even higher in production of thermomechanical pulp (TMP) or CTMP (chemo-thermomechanical pulp), which is chemically pretreated TMP. The temperature in the refiner is usually above 130.degree. C.
This temperature stress causes hydrolysis of some components of the wood, causing the wastewater to be heavily loaded with dissolved and colloidal compounds. It reaches a specific value of about 30 kg chemical oxygen demand per ton for groundwood, and up to more than 40 kg/ton for TMP. As the water loop is partially tightly closed, the loading in the circulating liquid is generally very high.
Low-molecular-weight compounds such as organic acids (e.g., acetic acid), sugar, short-chain hemicelluloses (e.g., arabinose), lignines (e.g., hydroxymatairesinol) and rosins (e.g., abietic acid) appear in the circulating water in dissolved or colloidal form. These are matters well known in the art.
In bleaching of wood pulps, the loading of the circulating water with these materials causes a serious deterioration of brightness and increases the need for chemicals. Washing the pulp is one possibility for improving the increase in brightness and decreasing the need for chemicals. That is done in many TMP plants, by diluting with large quantities of fresh water after the disintegration process and pressing it out again. The wastewater is not returned to the circulation, but goes directly to the wastewater treatment plant. Of course, that process uses a large amount of water. In many countries, though, fresh water is not available in unlimited amounts. Limits to the total chemical oxygen demand (COD) also reduce the possibility for wastewater cleanup by very high dilution. The difficulty of removing water from the pulp is a further problem. While chemical pulp can be dewatered and washed using relatively little water, by means of an appropriate complex technical system, such as pressure washing, that does not apply to the far more slimey mechanical wood pulp.
Wood pulp with a degree of beating of 70 to 80 Schopper-Riegler cannot practically be cleaned by diffusion washing. An effective washing process would require dilution and thickening and would be linked with a correspondingly high specific water consumption.
In the paper industry, the circulating water is cleaned by various mechanical and chemical-mechanical processes. While colloidal material can be removed only to a limited extent by filtration and sedimentation, it can be removed by total flotation using extremely long-chain polymers such as polyacrylamides as flocculating agents. These processes give nearly quantitative flocculation. The flocculated particles are separated from the circulating water by this total flotation and disposed of as sludge. The capital and operating costs of this circulating water cleanup process are a disadvantage, as is the complete loss of all the fibers in the circulation. Thus, an additional precleaning of the circulating water by a rotary disk filter is required if one wants to prevent fiber losses. This, again, makes the cleanup so expensive that usually only the main loop is subjected to such a process. Basically, though, it would be reasonable to do this cleanup step repeatedly so as to provide optimal conditions for the process steps and to reduce their chemical usage.
Accordingly, the industry has sought a process that reduces the need for bleach chemicals and may also improve the increase in brightness.